That's a shame, similar types of port and age appear to be going for £50 plus on ebay

If that's the case then you have something more special than a basic Sandeman Tawny bottled in 1990.

Tawny port is an entry level port. You can buy it today from supermarkets for around £10 per bottle. Tawny port is blended by the manufacturer to be drunk shortly after being bottled and is not intended to be aged - and actually deteriorates with age although rarely to the point where it is undrinkable.

There are variations on tawny port - 10 Year Old Tawny, 20 Year Old Tawny etc. These are more expensive than the basic Tawny as they are blended from older stocks of wine.

A picture would really help us to advise you what your options are, but if your bottle really is a basic Sandeman Tawny bottled 1990 and you see these selling on eBay for £50 then my strong advice would be to sell on eBay as someone is paying way above the market value for this bottle.